To suppress the atmospheric diffusion of particles contained in the exhaust gas of diesel engines, in recent years, various apparatuses are being developed for mounting in the exhaust system of diesel engines for after-treatment of diesel particulates in the exhaust gas. Exhaust gas treatment apparatuses of this type all include a filter for trapping particulates contained in the exhaust gas discharged from the diesel engine. Particulates trapped by the filter gradually accumulate and ultimately clog the filter, making it difficult for the exhaust gas to pass through the exhaust gas passage.
Conventionally, therefore, when it is estimated that the quantity of the accumulated particulates in the filter has reached a prescribed level, the filter is heated to elevate the filter temperature and burn the particulates, regenerating the filter so that it again can be used to trap particulates. It is therefore desirable to carry out filter regeneration at a suitable timing, using the means of burning the particulates by heating the filter.
Previously, wide use has been made of a filter regeneration control arrangement in which the quantity of accumulated particulates in the filter is estimated by measuring the before and after differential pressure of the filter, that is, the pressure differential between the exhaust gas pressure at the inlet end of the filter and the exhaust gas pressure at the outlet end of the filter, and determining the timing of the filter regeneration in accordance with the estimation result.
However, the above-described conventional technology has the following problem points. When the engine is operating at a low speed such as when it is idling or the like, the flow rate of the exhaust gas in the exhaust path decreases, which reduces the before and after differential pressure of the filter even when the quantity of accumulated particulates is small, making it difficult to estimate accurately the quantity of the accumulated particulates on the basis of the before and after differential pressure of the filter. Also, when the exhaust gas is in a low-pressure state, pressure sensor input/output characteristics that indicate the relationship between the exhaust gas pressure and the pressure sensor output signals are not linear. The precision of the pressure information possessed by the pressure sensor output signal therefore declines, and the level of the output signal from the pressure sensor also becomes lower and susceptible to noise, so pressure sensor characteristics problems also make it difficult to accurately estimate the quantity of accumulated particulates.
Furthermore, in cases such as when particulates have accumulated in the filter and the filter continues to be in a high-temperature state when engine operation has been stopped, volatile components released by the accumulated particulates in the filter produces cracking in the particulates. When such cracking occurs, exhaust gas passes through the cracked portion and inside the filter, reducing the air-flow resistance of the filter. As a result, the before and after differential pressure of the filter becomes smaller compared to before the occurrence of the cracking, and if in this state the exhaust gas differential pressure is used to estimate the quantity of accumulated particulates, it results in a reduced estimate of the quantity of accumulated particulates, making it difficult to accurately estimate the quantity of accumulated particulates.
As such, in previous apparatuses in which filter regeneration is controlled by measuring the before and after exhaust-gas differential pressure of the filter, regeneration is carried out in a state in which there is a large amount of accumulated particulates in the filter, which risks causing fusing damage to the filter. Due to the difficulty of accurately estimating the quantity of accumulated particulates, with higher filter regeneration frequency, there are also the problems of deterioration in fuel consumption and shortening of filter life.
An object of the present invention is to provide a filter control apparatus that is able to resolve the above-described problem points of the conventional technology.